At the time we were surprised the OS would indeed support 512 Mb RAM and 4 GB storage handsets, and NPU notes that that page made Microsoft’s strategy to provide updates for these handsets clear.

Point 2.3.4 notes:

It is not yet known of this if this will allow 4 GB Windows Phones like the Lumia 530 to be upgraded, but given the need for Microsoft to support Windows 10 Mobile on the cheapest devices to have a reasonable market share it seems likely that we will definitely see new Windows 10 Mobile handsets with 4 GB storage be released in the future.